1. Field of the Invention
The invention relates to epoxy adhesive or molding compound compositions suitable for connecting, assembling, encapsulating or packaging electronic devices particularly for display, circuit board, flip chip and other semiconductor devices. This invention particularly relates to one-part epoxy compositions that are of long shelf life at storage conditions but are highly reactive at the curing or molding temperature. More specifically, application of this epoxy relates to adhesives for an anisotropic conductive film (ACF) to adhesively bond two electric terminals to achieve good electroconductivity between the two terminals in the vertical Z-direction perpendicular to the adhesive film, while remain highly insulating within the horizontal X-Y plane of the film.
2. Description of the Related Art
Even though the one-part epoxy systems have the advantages of convenient applications as adhesive or molding compounds for connecting, assembling, encapsulating or packaging electronic devices, however, they still suffer severe drawbacks of low shelf-life stability and a requirement of storing at a low temperature. For adhesive or molding compound applications, the thermoset adhesives, particularly the epoxies, are considered as superior to the thermoplastic adhesives because of the process-ability of the uncured composition and the heat resistance of the cured products. Furthermore, among the epoxy applications, the one-part epoxy systems are in general more preferred than the two-part systems for most of the molding compounds and pre-coated products including anisotropic conductive adhesive films (ACFs or ACAFs). This is because the one-part systems are much more user friendly between these two systems.
Specifically, the one-part epoxy system typically includes an uncured epoxy resin component, a curing agent and/or an accelerator. The resin component includes at least one epoxy resin, which has two or more epoxy groups in a single molecule. The curing agent such as the catalyst or accelerator initiates and/or accelerates the reaction by either catalyzing and/or taking part in the reaction. Preferably, the accelerator component and the other epoxy adhesive components are selected such that the epoxy adhesive is very stable at the storage conditions but cures rapidly at the bonding temperature. Reviews of the epoxy crosslinking system may be found in, for examples, J. K. Fink, Reactive Polymers, Fundamentals and Applications_, William Andrew Publishing, NY (2005); J. A. Brydson, Plastic Materials, Ch. 26, 7th ed., Butterworth-Heinemann (1999); C. D. Wright and J. M. Muggee in S. R. Hartshorn, ed., Structure Adhesives, Ch. 3, Plenum Press, NY (1986); and H. Lee, The Epoxy Resin Handbook, McGraw-Hill, Inc., NY (1981). However, as disclosed in these publications, the one-part systems still suffer severe drawbacks due to the lack of shelf-life stability and the low temperature storage requirements.
The problem of shelf-life stability is even worse in applications that require high speed curing at low temperature. To address this issue, curing catalysts/accelerators are absorbed in a molecular sieve or in the form of microcapsules to enhance the curing processes as disclosed in Japanese Patent Publication No. 17654/68, 64/70523, U.S. Pat. Nos. 4,833,226, 5,001,542, 6,936,644. In the case of microencapsulated accelerators or curing agents, the microcapsules must be first broken or rendered permeable by pressure, shear, heat or combinations of above methods in order to cure the epoxy resin. Examples of commercially available imidazole microcapsules include the Novacure series from Asahi Chemical Industry Co., Ltd. However, in most cases, the stability is improved at the expense of curing speed.
The selection of the curing agents of the one-part epoxy system to assure shelf-life stability is very important for applying the one-part epoxy system as adhesive for making the ACF film. It has been found that the instability of ACF connection resistance may be attributed to the premature reaction between the adhesive and the curing agent that takes place during storage. Such premature reaction between the adhesive and the curing agent during storage causes a decrease of fluidity of the adhesives. The decrease in the adhesive fluidity further results in either (1) insufficiently removal of the adhesive, or the protective insulating shell, if present, from the surface of electroconductive particle or, (2) too high a viscosity of the adhesive before the conductive particles may come into a sufficient contact with circuit. It has also been found that such an insufficient contact is particularly remarkable when the electroconductive particle is a pressure-deformable particle. To suppress the undesirable reactions before bonding and improve the reliability of device connection of the high speed or low temperature curing ACFs, there is an urgent need for a more stable curing system while maintaining and even more preferably increasing the curing rate. However, until now, a curing agent/accelerator capsule with a thicker or less permeable wall when employed to improve the stability often achieve the stability improvement at the expense of the curing speed at the bonding conditions.
The use of leuco dyes in a variety of applications including carbonless paper, thermal recording paper and photoimaging systems has been reviewed extensively in for examples, R. Muthyala, ed., Chemistry and Applications of Leuco Dyes, Ch. 1, 4, 5 and 6 Plenum Press, NY (1997) and P. Gregory, High Technology Applications of Organic Colorants, Ch. 8, Plenum Press, NY (1991). In a conventional process for fabricating the anisotropic conductive film (ACF), the use of an irreversibly thermochromic pigment in the adhesive has been taught as a temperature indicator to show that the highest temperature has reached by a color change of the temperature indicator. However, such applications of various kinds of leuco dyes or pigments, particularly those disclosed in the present invention, do not address the technical difficulties of the shelf-life stability and the curing speed of the epoxy compositions particularly in applications such as connecting, assembling, encapsulating or packaging electronic devices.
Therefore, a need exists in the art to provide improved one-part epoxy adhesives, coatings or molding compounds particularly for manufacturing electronic devices with improved shelf-life stability and in the meantime provide high speed or low temperature curing process such that the above discussed difficulties and limitations can be resolved.